1. Field of the Invention
This invention relates to steel compositions intended in particular for the manufacture of intake and exhaust valves for vehicles that are powered by an internal combustion engine.
2. Description of Related Art
During use, this type of part is subjected to major mechanical stresses at temperatures that continue to rise as the power and performance of the engines in which they are installed increases. Currently, when the engine intake includes a turbo, this temperature is generally between 200 and 400xc2x0 C., although it can reach 800xc2x0 C. at the level of the exhaust when the fuel used is gasoline. The exhaust valves can therefore be subjected to temperatures that range up to 900xc2x0 C. for each ignition followed by an exhaust. The materials used for these valves must also be able to withstand sudden and large variations in temperature.
This increase in the temperatures of the valves during operation makes them even more sensitive to oxidation and corrosion caused by certain components of the fuels used, such as lead, sulfur and vanadium pentoxide, which reduce the useful life of the valves.
The direct oxidation of the metal represents the primary mechanism in the European countries, where regulations tend to require the use of unleaded gasoline and a reduction in the amount of sulfur in fuels to very low levels, on account of atmospheric pollution.
Apart from these various stresses that are encountered during the utilization of the finished parts, the steel or alloy used to manufacture them must also satisfy certain additional criteria. The valves are generally manufactured in two stages. First of all, the steelmaker will prepare a grade of steel or alloy which it will then supply to the valve manufacturer in the form of bars that have been straightened, but which may also have been rough turned or subjected to any other surface treatment specified by the customer. This manufacturer will then proceed to shear these bars, in an operation that is also called blank cutting. In an initial manufacturing process, the bar is cut into blanks at a high temperature, which is followed by the extrusion of the blanks into valves at temperatures ranging from 1150 to 1200xc2x0 C., which requires that the grain structure of the bar supplied remain stable up to the forging temperatures.
In a second manufacturing process, called upsetting, the blanks are obtained by shearing at the ambient temperature, which requires a metal that is not very brittle, to prevent non-uniform shearing and the cracking of these blanks. It has also been found that during this cold shearing operation, problems can occur that are related to carbide segregations in the blanks, which can result among other things in excessive wear of the tools.
The steels of the prior art cause problems during shearing because, among other things, the appearance of cracks in the parts requires frequent adjustments to the production lines.
The materials conventionally used for the manufacture of valves of this type include austenitic stainless steels, which have an iron-nickel-chromium base and range from steels with a high manganese content (up to 10% by weight) to steels with a high nickel content (up to 21% by weight). The high-temperature oxidation resistance of these steels is not always satisfactory, in particular when, for example, the engine is operating in a marine atmosphere and is exposed to chlorine, or when an increase in the performance of the engine results in hotter combustion gases. These insufficiencies have led steelmakers to increase the chromium content of their steels even further, which has the disadvantage that it promotes the formation of ferrite at high temperature, as well as intermetallic phases that make the steel brittle at engine operating temperatures.
The essential object of this invention is therefore to eliminate the above mentioned disadvantages of the steel compositions of the prior art by making available steel compositions that have, among other things, improved resistance to oxidation, improved mechanical characteristics and improved operational properties that make it possible, among other things, to manufacture exhaust valves that have excellent mechanical strength and resistance to oxidation in the temperature range from 800 to 900xc2x0 C.
For this purpose, a first object of the invention consists of a steel composition which contains, expressed in percentages by weight:
whereby C+Nxe2x89xa70.8%,
and the rest consists primarily of iron and the unavoidable impurities.
In one preferred embodiment of the invention, the steel composition includes, expressed in percentages by weight:
whereby C+Nxe2x89xa70.9%,
and the rest consists primarily of iron and the unavoidable impurities.
The inventors have discovered, to their surprise, that the steel compositions defined above all have a solidification mode that is very close to a eutectic between the y phase of austenite and a phase which has been found to he a niobium carbonitride Nb(C,N).